1. Field of the Invention
The present invention relates to an anode material enabling production of a lithium secondary battery having a large capacity, a high voltage and excellent charge-discharge cycle property and free from decomposition of solvents of electrolytic solution; a process for production of the anode material; and a lithium secondary battery containing the anode material.
2. Description of the Related Art
As electronic appliances have become smaller and lighter, the batteries used therein are required to have a higher energy density. It is also required to develop a high-performance secondary battery allowing repeated charge and discharge, from the standpoint of resource saving. In order to respond to these requirements, lithium secondary batteries of high energy density, light weight, small size and excellent charge-discharge cycle property have been proposed.
Lithium secondary batteries are classified, depending upon the kind of the electrolyte used therein, into lithium-ion secondary battery, lithium polymer battery, completely solid-state lithium secondary battery, etc. Of these secondary batteries, lithium-ion secondary battery is drawing particular attention for solving the problems of lithium metal secondary battery such as low charge speed, short cycle life, low safety and the like, and research and development are being continued on lithium-ion secondary battery.
Lithium metal secondary battery uses lithium metal for the anode. In contrast, lithium-ion secondary battery uses a carbon material for the anode and, by using a carbon material for the anode, it is intended to satisfy the above-mentioned requirements. That is, in lithium-ion secondary battery whose cathode is constituted by a lithium compound and whose anode is constituted by a carbon material, when the battery is in a charge stage, the carbon material is doped with lithium ion at the anode, whereby a so-called carbon-lithium intercalation compound is formed. Meanwhile, when the battery is in a discharge stage, lithium ion is undoped from between the adjacent layers of the carbon material, and the undoped lithium ion migrates back to the cathode and returns to the lithium compound. Owing to such a mechanism, lithium-ion secondary battery enables repeated charge and discharge.
As the anode used in lithium-ion secondary battery, there are a graphite type anode and a carbon type anode. When a battery produced using one of such anodes is subjected to long-term discharge in disregard for practical usability, the discharge capacity of the carbon type anode is 600 mAh/g or more. This discharge capacity is large as compared with the discharge capacity (350 to 370 mAh/g) of the graphite type anode in practical use. The discharge capacity of the carbon type anode in practical use, however, is 250 to 300 mAh/g and is small as compared with the discharge capacity (350 to 370 mAh/g) of the graphite type anode in practical use. Further, the carbon type anode is low in density (of the carbon material used therein) and also in discharge voltage. Therefore, a battery using the carbon type anode is inferior also in discharge energy, as compared with a battery using the graphite type anode. For the above-mentioned reasons, the graphite type anode is used in many lithium-ion secondary batteries.
In lithium-ion secondary battery using a graphite type anode, it is desired for efficient charge and discharge in a short time that the graphite constituting the graphite anode is in the form of fine particles. It is thought that when the graphite is in the form of fine particles, the contact area between graphite and electrolytic solution in battery is larger and, as a result, the transfer of lithium ion between electrolytic solution and graphite is easier.
The present inventors made a study on the use of fine graphite particles for the anode of lithium-ion secondary battery. As a result, an increase in discharge capacity of battery was confirmed by the use of fine graphite particles. However, the use of fine graphite particles brought about an increase in the charge amount necessary for battery operation and a resultant decrease in coulombic efficiency. Further, the use of fine graphite particles increased the reactivity of the graphite with the solvent constituting the electrolytic solution of battery, which invited decomposition of the solvent and generation of striking amount of gas. The generation of gas in closed battery incurs an increase in the internal pressure of battery and a high risk of battery explosion. Therefore, it is extremely important to suppress the generation of gas inside battery.
In general, the main solvent of the electrolytic solution used in lithium-ion secondary battery is, in many cases, a carbonic acid ester such as ethylene carbonate (hereinafter referred to as EC), propylene carbonate (hereinafter referred to as PC) or the like. This main solvent is mixed with an electrolyte such as LiPF6, LiBF4 or the like, whereby an electrolytic solution is obtained. The reason why a solvent such as PC, EC or the like is used as the main solvent of electrolytic solution is that these solvents have desired solvent properties such as high relative dielectric constant, operability in wide temperature range and the like. PC, in particular, is a solvent usable at low temperatures. However, when an electrolytic solution containing PC is allowed to co-exist with a graphite type anode in a battery, PC is decomposed and generates a gas, as mentioned above. This decomposition of PC is seen only when graphite is used as the anode of battery, and is not seen when a carbon type anode is used.
As mentioned above, coexistence of PC-containing electrolytic solution and graphite type anode in battery gives rise to PC decomposition. As a result, the coulombic efficiency of battery decreases; moreover, the internal pressure of battery increases, leading to a high risk of battery explosion. Hence, it is desired to develop such a graphite type anode that gives rise to no PC decomposition when a PC-containing electrolytic solution is used and that causes no reduction in coulombic efficiency, i.e. a graphite type anode highly resistant to PC decomposition.
There have hitherto been proposed methods for suppressing PC decomposition by covering the surfaces of graphite particles with low-crystalline carbon which causes no PC decomposition.
As examples of such methods, there are mentioned a method (disclosed in Japanese Patent No. 2643035) of using, as an anode material, a composite material obtained by covering graphite with low-crystalline carbon by chemical vapor deposition; a method (disclosed in Japanese Patent Application Laid-open Hei-5-121066) of using, as an anode material, a composite material obtained by covering graphite with carbon having an average layer spacing d002 of 0.337 nm or more; and a method (disclosed in Japanese Patent Application Laid-open Hei-5-275076) of using, as an anode material, a composite material obtained by covering graphite with amorphous carbon.
The anodes produced using one of the above composite materials can suppress PC decomposition. However, a battery using such an anode produced from a composite material, as compared with a battery using an anode produced from graphite particles alone, has problems possessed by low-crystalline carbon, such as small discharge capacity in practical use, low speed in charge and discharge, and the like.
The carbon type anode or the graphite type anode, each used in lithium-ion secondary battery has problems possessed by the carbon type material used in the former anode, or problems possessed by the graphite type material used in the latter anode; and these problems run counter to each other. Hence, it is desired to develop an anode material for lithium-ion secondary battery capable of solving all of the above problems, and a lithium-ion secondary battery using such an anode material.
The present inventors examined the usability, as an anode material for lithium-ion secondary battery, of a graphite-carbon composition material obtained by covering graphite with carbon by chemical vapor deposition under various conditions. As a result, it was found out that when there is used, as an anode material for lithium-ion secondary battery, a graphite-carbon composite material obtained by covering the surface of graphite with crystalline carbon uniformly and completely, the anode material can reliably suppress the decomposition of PC or the like; the battery using the anode material is high in discharge capacity and allows rapid charge; the anode material has electrode performances superior to those of conventional anode materials covered with low-crystalline carbon. It was further found out that the above anode material is usable in any of lithium secondary batteries such as lithium polymer secondary battery, solid-state lithium secondary battery, lithium-ion secondary battery and the like.
It was furthermore found out that when lithium ion is intercalated into the above anode material and the resulting material is measured for 7Li-NMR spectrum, the spectrum can be used for measurement of the crystallinity of the above anode material. These findings have led to the completion of the present invention.
The present invention aims at providing an anode material capable of producing a lithium secondary battery which alleviates the above-mentioned problems of the prior art, which suppresses the decomposition of solvent of electrolytic solution, and which is high in discharge capacity and enables rapid charge and discharge; a process for production of such an anode material; and a lithium secondary battery produced using such an anode material.
The present invention lies in the followings.
[1] An anode material for lithium secondary battery, comprising graphite particles and a crystalline carbon layer covering the whole surfaces of the graphite particles, wherein the whole surfaces of the graphite particles are covered with a carbon layer in a state that the surfaces of the graphite particles and the carbon 002 plane of the carbon layer are parallel.
[2] An anode material for lithium secondary battery according to [1], which has absorption spectra at 40 to 50 ppm and 10 to 30 ppm when lithium ion is intercalated thereinto and the resulting material is measured for 7Li-NMR spectrum using lithium chloride as a standard.
[3] An anode material for lithium secondary battery according to [1], wherein the carbon layer shows optical anisotropy under a polarizing microscope.
[4] An anode material for lithium secondary battery according to [1], wherein natural graphite is used as the graphite particles.
[5] A lithium secondary battery produced using an anode material set forth in any of [1] to [4].
[6] A process for producing an anode material for lithium secondary battery, comprising graphite particles and a crystalline carbon layer covering the surfaces of the graphite particles, which process comprises subjecting graphite particles to a treatment for chemical vapor deposition using an organic substance gas or a mixed gas consisting of an organic substance gas and an inert gas, in a fluidized bed type reactor to form a carbon layer on the surfaces of the graphite particles.
[7] A process according to [6], wherein the carbon layer is a crystalline carbon layer and the whole surfaces of the graphite particles are covered with the carbon 002 plane of the crystalline carbon layer.
[8] A process according to [6], wherein the molar concentration of the organic substance gas in the mixed gas is 2 to 50% and the temperature of the treatment for chemical vapor deposition is 900 to 1,200xc2x0 C. 
[9] A process according to [6], wherein natural graphite is used as the graphite particles.